Method for the rastered reproduction of half-tone pictures

ABSTRACT

A method for the raster reproduction of half-tone pictures wherein a picture is scanned photoelectrically dot-by-dot and voltage values are generated and quantified to represent brightness ranges and wherein an alternating voltage is superimposed on the voltage values before they are quantified to generate a mixing of dot sizes indicative of one brightness range with dot sizes indicative of another adjacent brightness range and to cause the mixing to occur in such a pattern as to make transition between brightness ranges to be more pleasing to the eye.

United States Patent K01! 1 51 Aug. 1, 1972 [54] METHOD FOR THE RASTERED[56] References Cited REPRODUCTION OF HALF-TONE PICTURES UNITED STATESPATENTS Z t [72] inventor: Roman Koll, Kiel-Weliingdorf, Ger- 3'393'2697/1968 eu 178/6 6 8 many Primary Examiner-Benjamin A. Borchelt [73]Assignee: Dr.-Ing. Rudolf Hell Assistant f Kmber3 Attorney-Hill,Sherman, Merom, Gross & Simpson [22] Filed: May 2, 1969 211 App]. No.:821,380 [57] ABSTRACT A method for the raster reproduction of half-tonepictures wherein a picture is scanned photoelectrically [30] ForeignAmman mm dot-by-dot and voltage values are generated and quan- May 4,1968 Germany ..P 17 72 367.0 9 represent ghtness ranges and wherein analternating voltage IS superimposed on the voltage values before theyare quantified to generate a mixing [52] [1.8. CI ..3l5/30, l78/6.7 ofdot sizes indicative of one brightness range with dot [51] Int. Cl...H01J 29/52 sizes indicative of another adjacent brightness range [58]Field of Search...3l5/30; 178/67, 6.6 B, BIG 3 and to cause the mixingto occur in such a pattern as to make transition between brightnessranges to be more pleasing to the eye.

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PKTENTEU 1 I973 SHEET 2 BF 4 llllllh METHOD FOR THE RASTEREDREPRODUCTION OF HALF-TONE PICTURES BACKGROUND OF THE INVENTION Theinvention relates to a method for raster reproduction of half-tonepictures which have high quality in color density gradations and whichonly requires the usual number of raster dot sizes, normally associatedwith an inferior quality of reproduction.

It is well known that in order to reproduce half-tone pictures in reliefor offset print, the printers forms must be rastered since it is notpossible to reproduce the half-tones by suitably adjusting the thicknessof the printing ink. Density gradations are obtained by using differentsize raster dots.

Where the printers forms are produced by the chemical etching process,the raster is produced through photomechanical means by photographicallycopying a transparency or negative of the picture together with acontact raster film. The raster areas in this type of contact raster arevignetted, i.e. the degree of blackness decreases from a maximum at thecenter of the raster field towards the edge. The material used forcopying in such cases is a film which is very hard," that is to say,which has a very sharply rising density characteristic. By vignettingthe contact raster in conjunction with the sharp sensitivity thresholdof the film it is possible to determine the size of each raster dot fromthe local density value of the half-tone picture.

It is well known that printer's forms for relief printing may also beproduced using engraving machines, the depth of penetration of theengraving stylus being controlled in dependence upon the brightness ofan original which is photoelectrically scanned. The raster is achievedby means of a raster frequency current which is superimposed on theengraving stylus control current.

In each of the raster forming methods described above, a defined size ofraster dot results from each degree of density of the half-tone picturewhich size varies continuously with the density.

It has recently become necessary to compose halftone pictures inrastered form using electronic phototype letters in which the elementsof the picture are stored in quantified and binary coded form and areread out to control an electron beam which traces the picture to bereproduced on a screen. If a rastered picture is to be traced, thepicture content of the raster dots of different sizes must be readilyavailable, for example, in a ring core store.

In order to keep the store capacity to be used for this purpose withinreasonable limits, the number of quanta or the number of differentraster dot sizes must be kept very small. The number of quanta regardedas a minimum for reproduction of any accuracy is, as known from thecommunications art, 32.

A relatively rough gradation of this type may be sufficient for portionsof the picture which are highly structured but in sections which are notso highly structured, that is to say, in areas where the density onlyvaries very gradually, the quality of reproduction leaves much to bedesired. In these areas the density gradations show as equally densezones which are markedly shifted relative to one another. This is knownto be disturbing to the eye.

SUMMARY OF THE INVENTION It is an object of the invention to makepossible the reproduction of pictures without gradations in densitytransition and without increasing the number of raster dot sizes whichhave to be stored.

According to the invention, in order to achieve those density valueswhich lie between any two adjacent density gradations which correspondto the specified raster dot sizes, raster dots of different, butpreferably adjacent sizes are mixed with a statistical distribution,particularly in a proportion which corresponds to the ratio of thedifi'erenoes between the density value to be achieved and the twoadjacent density gradations.

There exists conventional machines in which the original picture isscanned dot-by-dot photoelectrically and the voltage values of thepicture signal analogous with the brightness values are quantified, i.e.assigned at regular moments in time determined by a timer each to one ofa number of discrete voltage values which correspond to the differentraster dot sizes and are expressed in binary code. In such devices, themethod of the invention may be used by superimposing an alternatingvoltage on the picture signal voltages before the process ofquantification, the amplitude of the said alternating voltage preferablybeing equal to half the difference between two successive, discretevoltage values while its frequency is not a harmonic in relation to thefrequency of the timer.

The frequency of this alternating voltage is advantageously at least aquarter of the timer frequency.

It has proved advantageous from the point of view of statisticaldistribution of different raster dots which it is desired to achieve, touse a triangular or sawtooth alternating voltage as the voltage to besuperimposed on the picture signal.

It is a well known fact that the human eye sees many fewer contrasts inthe light portions of a picture than it does in the dark ones. Thus, inorder not to have too few density gradations in the light portions andtoo many in the dark portions, there will be no uniform gradation butthe discrete voltage stages will increase with increasing density, inaccordance with the well known physiological sensitivity curve (Munsellfunction of like physiological differences in sensitivity). Consequentlythe amplitude of the voltage for superimposition will also be increasedas density increases.

It is impossible to express the amplitude of the superimposed voltageanalytically since the physiological sensitivity curve is merely anempirical function. In general all that can be said is that theamplitude of the superimposed voltage follows another non-linearfunction. its real development can, however, be determined in actualcases without difficulty from the condition already mentioned, viz. thatit is preferably equal to half the quantum stage, in this case thereforethe quantum stage passed at a time.

in order to achieve a non-linear increase in the superimposed voltage,the invention further proposes that the amplitude of the superimposedvoltage be rendered dependent upon a voltage obtained by a nonlineardistortion of a voltage proportional to the density of the picture.

The quantified and binary coded voltage values are then either usedimmediately to control the reproduction of a picture or are stored on arecord carrier such as a punched tape from which it can be read out atany subsequent date. The data for the picture contents of the variousraster dot sizes may be stored in ring core stores and the reproductionof the picture consists in recalling the raster dot sizes required whenthe picture was recorded from the ring core store using the data readoutof the record carrier and in using them to control the recording of thepicture.

The method for the raster reproduction of half-tone pictures comprisesthe steps of forming raster dots of a first size in correspondence witha first tonal value which is representative for a first range of tonalvalues of a tone control scale, forming raster dots Of a second size incorrespondence with an adjacent second tonal value which isrepresentative for a second range of tonal values of a tone controlscale, interrnixing said first and second size raster dots if the tonalvalue to be achieved lies between said first and second tonal values,and reproducing said intermixed first and second size raster dots. Morespecifically this is accomplished by photoelectrically scanning ahalf-tone picture dot by dot, generating voltage values, quantifyingsaid values to represent brightness ranges, and superimposing analternating voltage on said voltage values before they are quantified togenerate a mixing of dot sizes indicative of one brightness range withdot sizes indicative of another adjacent brightness range and causingthe mixing to occur in such a pattern as to make the transition betweenbrightness ranges more pleasing to the eye. The mixing of the dots maybe best illustrated with the following example. Thus, if a brightness ordensity value has been obtained during the scanning process which liesbetween the median tone values of the individual tone-value ranges, amixing of the raster dots will be effected during the recordationwhereby the raster dots which correspond to the higher tone value of thehigher tone value range will be mixed with the lower tone value of thelower tone value range. Depending on whether the scan brightness ordensity value is near to the upper tone value or to the lower tone valuemedian, the mixture ratio of the raster dots will be favorable for thehigher or lower tone value, i.e. to the raster dot sizes which areassigned to those tone values. Let us assume that the density orbrightness values which correspond to the tone values of 17.5 percent,20, 21.25 and 22.5 percent have been obtained during the scanningprocess. With the tone value of I75 percent, 100 percent raster dots ofthe tone value of 17.5 percent and percent of the raster dots having thetone value of 22.5 percent will be printed. When we have a tone value ofpercent there will be a mixture of 50 percent raster dots of the tonevalue of 17.5 percent and 50 percent of the tone value of 22.5 percent.Similarly, the tone value 21.25 percent will be reproduced by means ofpercent raster dots of the tone value of 17.5 percent and 75 percent ofthe tone value of 22.5 percent. For a tone value of 22.5 percent [00percent raster dots of the tone value of 22.5 percent will be printed.

Other objects, features and advantages of the invention will be readilyapparent from the following description of a preferred embodimentthereof, taken in conjunction with the accompanying drawings, althoughvariations and modifications may be effected without departing from thespirit and scope of the novel concepts of the disclosure, and in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. la shows a three-stage grey wedgebuilt up from quantified raster dots.

FIG. lb shows a similar grey wedge which is, however, formed accordingto the invention by mixing the raster dots.

FIG. 2 shows a diagram of the superimposing of three equal quantumstages.

FIG. 3 shows the superimposing of a triangular voltage with an amplitudewhich increases when the quantum stages increase.

FIG. 4 shows a circuit diagram of a system for producing the triangularvoltage of FIG. 3.

FIGS. 5 and 6 show examples of different superimposed voltages.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIGS. la and lb, the areamarked m is a medium shade of grey, the area marked m l is the nextlightest and the area marked m l the next darkest.

If it be assumed that the raster dot size in each case represents theshade of grey which occurs in a nongraduated grey wedge in the center ofthe area 1 or 2 or 3, in FIG. la, each area on the righthand boundary isthus half a shade too light and on the lefthand boundary half a shadetoo dark. In the boundary areas 4 and 5 there thus occurs, according toFIG. la, a sharp change in density. This change has been eliminated inFIG. lb by causing each of the two vertical lines of raster dots tocontain equal proportions of raster dots of the lighter and darker shadewhich are, moreover, differently distributed in the two rows. As will bedescribed in more detail below, this is a statistical distribution. Theresult is a grey shade which, as was intended, corresponds to theaverage of the values of the grey shades involved.

The continuous transitions between the middle and the limits of therange are achieved in a similar manner, viz. by reducing as continuouslyas possible the portion of inserted raster dots of an adjacent greyshade from the end of the range towards the middle of the range.

When assessing the effect of FIGS. la and 1b, it should be rememberedthat these show the raster dots substantially magnified. In practice thelargest raster in use is, as is well known, 400 raster dots per squarecentimeter. Even here, the individual raster dot is so small that thehuman eye is incapable of resolving a picture which has been so rasteredinto the individual raster dots. In order to reconstruct theseconditions it would be necessary to view the Figures from a distance ofseveral meters.

The above remarks made in connection with the grey shades also apply forthe saturation values of colors.

In the diagram given in FIG. 2, the values on the ordinate are voltagevalues E which correspond to degrees of blackness. In the diagram, E 0is appropriated to the value white. The abscissa is the time axis, onwhich are marked the scanning clock pulses.

By means of lines 10, ll, 12 and 13 there are marked on the ordinatevoltage ranges of equal size, of which it is intended to consider theranges m I m and m I A particular individual voltage value is associatedwith each of these ranges, and in the present case this is always theaverage value in the range. This value is recorded as long as thepicture signal voltages which occur are located within the rangeconcerned. The average value of the range is shown in the drawing ineach case by a horizontal dotted line.

Let us assume that a portion of the picture being scanned becomesgradually darker. This produces a picture signal voltage which may berepresented by a rising straight line 14. As a result of thequantification and without further measures, there would occur at thelocations at which the straight line 14 intersects one of the rangelimits, e. g. 11 and 12, sudden variations in the degree of blacknesssuch as are found in FIG. la. The result of the superimposition, on thepicture signal voltage, of an alternating voltage which in the presentcase is a triangular voltage, is that the resultant voltage which is tobe quantified, which follows the line has, for certain scanning clockpulses, a momentary tonal value which lies in the quanta rangeimmediately above or immediately below that which passes through thepicture signal voltage range.

In FIG. 2 all the scanned momentary tonal values in the range m areindicated by small circles and all those which occur, for one of thescanning clock pulses in the range m l or m l are indicated by smallcrosses.

If, as is here the case, the peak-to-peak rise of the alternatingvoltage (double amplitude) is equal to the quanta shade voltage, thefollowing is true: If the picture signal voltage passes straight throughthe center of a range (dotted line), all scanned momentary values fallcorrectly into this range. If, on the other hand, it passes straightthrough a range limit, e.g. 1 l, the scanned momentary tonal values liein equal portions in the range through which the voltage has passed andin the adjacent range, e.g. m l and m. [f the picture signal voltageapproaches the center of the range, the portion of the momentary valueswhich lie in this range during scanning increases steadily.

The size of the peak-to-peak rise of the alternating voltage mentionedabove represents an optimum. Thus, if this rise is smaller than thequanta shade voltage, the mixture of raster dots becomes imperfect. If,on the other hand, it is larger, raster dots from a non-adjacent rangewill be mixed in, and this results in a certain coarseness in thepicture as reproduced.

As already mentioned, the frequency of the scanning clock pulses t, andthe frequency of the superimposed voltage should not be harmonic. Thisavoids a periodic repetition of the raster clot distribution and ensuresinstead that the distribution is statistical, as when a random generatoris used.

Instead of superimposing a triangular voltage as shown, it is equallypossible to superimpose a sawtooth voltage. The only condition is thatthe alternating voltage used should have straight flanks and as far aspossible no horizontal curved portions.

in the diagram of FIG. 3, the ordinate is divided into increasingvoltage ranges corresponding to the physiological curve mentioned, i.e.m 4 m 3 and the picture signal voltage 14 has superimposed on it analternating voltage 16, the amplitude of which increases as a functionof the size of the picture signal voltage. The degree of this dependenceis chosen so that the peak-to-peak rise of the alternating voltage isalways approximately equal to the voltage range through which thepicture signal voltage passes. If the picture signal voltage passesbeyond a range limit, the peak-to-peak rise assumes correspondingintermediate values. FIG. 3 shows a voltage variation which takes intoaccount the well known Munsell physiological sensitivity curve of thehuman eye and FIG. 2 is a system of curves in which the Munsellphysiological sensitivity curve of the human eye is not taken intoaccount. The amplitude values of the voltage in FIG. 3 show clearly thecourse of this sensitivity curve.

FIG. 4 shows a circuit arrangement which enables the above-describedtriangular voltage of variable amplitude to be produced.

The inverted (increasing with the degree of blackness) picture signalvoltage E, is applied to the terminals 21 and 22 so as to be balanced toground.

The voltage applied to the terminal 23 is a rectangular voltage which isconveyed to the bases of the two transistors 24 and 25. Since thetransistor 24 is an npntransistor and the transistor 25 is apup-transistor, the transistor 25 is blocked when the transistor 24 isgated, and vice versa.

As soon as the transistor 24 becomes conductive. the capacitor 26 beginsto be charged through this transistor, and via the resistors 27 and 28.By suitably choosing the resistance values and the capacitor value it ispossible to determine the time constant for the charging process so asto make it large in relation to the gating time. This means that onlythe first, still approximately linear, portion of the charging curve isused.

If the transistor 24 is blocked again and the transistor 25 gatedinstead, the capacitor 26 is re-charged via the resistors 29 and 30.When this is effected with the same time constant, a symmetricaltriangular voltage is produced on the capacitor 26. This triangularvoltage is first amplified by means of the amplifier 31 and then passedthrough the transformer 32 to the line 33, so that the picture signalvoltage on which the triangular voltage has been superimposed isavailable on the terminal 34.

Since it is desirable that a not inconsiderable superimposing voltage beavailable when the picture signal voltage becomes very small (whitevalue), the auxiliary voltage sources 35 and 36 have been provided whichsupply the capacitor 26 via the resistors 37 or 38.

Not only the physiological sensitivity curve, but also the curverepresenting the increase in the amplitude of the superimposed voltageis non-linear. It is thus necessary to produce a non-linearity betweenthe picture signal voltage Ea or Ea and the amplitude of thesuperimposed voltage. This is effected by means of the diodes 39 and 40which are connected in parallel each to one of the resistors 28 and 30.Because of the shape of the diode characteristics, the influence of thisshunt will be slight when the value of the voltage Ea is low, but asthis voltage Ea increases it will increase to such a degree that finallythe resistors 28 and 30 are practically short-circuited.

When a sawtooth voltage is desired instead of a symmetrical triangularvoltage, the time constants, i.e. the resistances of the resistors 27,28 and 29, 30 and also the gating ratio of the rectangular voltage whichcontrols the transistors 24 and 25 must be changed in a well knownmanner to effect the desired time constants.

FIG. shows triangular voltages 41, 42, 43 and FIG. 6 sawtooth voltages44, 45, 46 of varying amplitudes such as occur for various picturesignal voltage values.

The invention claimed is:

l. A method for the raster reproduction of halftone pictures comprising:scanning the picture dot-by-dot photoelectrically and quantifying thevoltage values of the picture signal analogous with the brightnessvalues of the picture being reproduced, assigning different raster dotsizes to each quantified voltage value and expressing the same in binarycode, superimposing on the voltage values before the process ofquantification an alternating voltage. v

2. A method in accordance with claim 1 including forming the amplitudeof the alternating voltage to be approximately equal to half thedifi'erence between two successive quantified voltage values.

3. A method as claimed in claim 2, including timing the points at whichthe voltage values are quantified and forming the alternating voltage tohave a frequency which is at least one-quarter of the timing frequency.

4. A method as claimed in claim 2 wherein the alternating voltage is atriangular-shaped voltage.

5. A method as claimed in claim 2 wherein the alternating voltage whichis superimposed is a sawtoothshaped voltage.

6. A method as claimed in claim 2 wherein the quantified voltage stagesincrease with increasing density in accordance with the Munsellphysiological sensitivity curve, and wherein the amplitude of thealternating voltage for superimposition also increases with increasingdensity.

7. A method as claimed in claim 6, including forming the amplitude ofthe superimposed voltage to be a function of a voltage which is obtainedby a non-linear distortion of a voltage proportional to the density ofthe picture.

8. A method for the raster reproduction of half-tone pictures comprisingthe steps of: forming raster dots of a first size in correspondence witha first tonal value which is representative for a first range of tonalvalues of a tone control scale, forming raster dots of a second size incorrespondence with an adjacent second tonal value which isrepresentative for a second range of tonal values of a tone controlscale, intermixing said first and second size raster dots if the tonalvalue to be achieved lies between said first and second tonal values,reproducing said intermixed first and second size raster dots.

9. A method as claimed in claim 8 wherein the raster dots of said firstand second sizes are mixed in a proportion which corresponds to theratio of the differences between the tonal value to be achieved and saidtwo adjacent first and second tonal values.

10. A method for the raster reproduction of half-tone picturescomprising the steps of photoelectrically scanning a half-tone picturedot by dot, generating voltage values, quantifying said values torepresent brightness ranges, and superimposing an alternating voltage onsaid voltage values before they are quantified to generate a mixing ofdot sizes indicative of one brightness range with dot sizes indicativeof another adis ssar ias. as:saiaws sssms as ferences between the valueto be achieved and the adjacent values.

1. A method for the raster reproduction of halftone pictures comprising:scanning the picture dot-by-dot photoelectrically and quantifying thevoltage values of the picture signal analogous with the brightnessvalues of the picture being reproduced, assigning different raster dotsizes to each quantified voltage value and expressing the same in binarycode, superimposing on the voltage values before the process ofquantification an alternating voltage.
 2. A method in accordance withclaim 1 including forming the amplitude of the alternating voltage to beapproximately equal to half the difference between two successivequantified voltage values.
 3. A method as claimed in claim 2, includingtiming the points at which the voltage values are quantified and formingthe alternating voltage to have a frequency which is at leastone-quarter of the timing frequency.
 4. A method as claimed in claim 2wherein the alternating voltage is a triangular-shaped voltage.
 5. Amethod as claimed in claim 2 wherein the alternating voltage which issuperimposed is a sawtooth-shaped voltage.
 6. A method as claimed inclaim 2 wherein the quantified voltage stages increase with increasingdensity in accordance with the Munsell physiological sensitivity curve,and wherein the amplitude of the alternating voltage for superimpositionalso increases with increasing density.
 7. A method as claimed in claim6, including forming the amplitude of the superimposed voltage to be afunction of a voltage which is obtained by a non-linear distortion of avoltage proportional to the density of the picture.
 8. A method for theraster reproduction of half-tone pictures comprising the steps of:forming raster dots of a first size in correspondence with a first tonalvalue which is representative for a first range of tonal values of atone control scale, forming raster dots of a second size incorrespondence with an adjacent second tonal value which isrepresentative for a second range of tonal values of a tone controlscale, intermixing said first and second size raster dots if the tonalvalue to be achieved lies between said first and second tonal values,reproducing said intermixed first and second size raster dots.
 9. Amethod as claimed in claim 8 wherein the raster dots of said first andsecond sizes are mixed in a proportion which corresponds to the ratio ofthe differences between the tonal value to be achieved and said twoadjacent first and second tonal values.
 10. A method for the rasterreproduction of half-tone pictures comprising the steps ofphotoelectrically scanning a half-tone picture dot by dot, generatingvoltage values, quantifying said values to represent brightness ranges,and superimposing an alternating voltage on said voltage values beforethey are quantified to generate a mixing of dot sizes indicative of onebrightness range with dot sizes indicative of another adjacentbrightness range and mixing the dot sizes in a proportion whichcorresponds to the ratio of the differences between the value to beachieved and the adjacent values.